Field of Invention
The present invention relates to vehicle transmissions and in particular, but not exclusively, to a transmission adaptable for use with a variety of configurations of agricultural vehicles, such as tractors.
Description of Related Art
Transmissions form part of the driveline tractors. The purpose of the driveline is to transmit torque from the engine (most usually an internal combustion engine and/or or electric motor) to the wheels (or tracks as may be the case in larger tractors). In addition to the engine and transmission, the driveline may also include a flywheel; one or more clutches; a transfer box; and front, centre and rear differentials.
The configuration of the driveline depends on the specific type of tractor, for example whether the vehicle is rear wheel drive or four wheel drive, front wheel steered or centrally articulated, tracked or provided with conventional wheels and tyres. Each variation of driveline requires a different transmission layout.
Additionally, the transmission may be required to drive front and/or rear power take off (PTO) shafts to allow the tractor to operate ancillary equipment such as seed drills and bailers.
By way of an example, a rear wheel drive tractor with steered front wheels would not require a front differential or a front output shaft from the transmission, but would require a rear output shaft and a rear differential. In addition it may be provided with front and/or rear PTO shafts.
In contrast, a tracked tractor would not require an output shaft from the transmission to the front wheels, but would require a rear drive shaft and rear differential capable of splitting the torque between the left and right tracks in order to steer the tractor. Furthermore, the rear output shaft of a tracked tractor would be repositioned with respect to the tyre tractor due to the smaller size of the wheels driving the track.
Historically, the provision of different transmissions for each configuration of vehicle has necessitated the design and development of a distinct transmission for each particular application. This has led to the parallel development of transmissions for each of the following commonly provided configuration of tractor:                a) standard tractors with undriven and steered front wheels of smaller diameter and driven rear wheels of larger diameter;        b) four wheel drive standard tractors with steered and driven front wheels of smaller diameter and driven rear wheels of larger diameter; and        c) tracked tractors where only the rear track wheels, not the front idler wheels, are driven whereby the rear track wheels are at a lower level relative to the chassis.        
It will therefore be appreciated that there is at present a considerable duplication of effort in designing and developing multiple variations of transmissions to fit the various configurations of vehicle.
Different configurations of vehicle require the output shafts to be positioned at differing heights relative to the input shaft. Furthermore, transmissions for agricultural vehicles such as self-propelled harvesting or application machinery may require an even greater variation in shaft vertical position than that required across the range of tractor configurations listed above.
Unlike passenger cars and trucks where the transmission is mounted on a chassis, in agricultural vehicles the transmission is often an integral (and structural) part of the chassis. As a result the space envelope for the transmission is severely restricted by the transmission casing which forms an integral part of the chassis. Furthermore, since the transmission is part of the structure of the chassis, each variation in transmission design requires structural testing thereby increasing the cost and complexity of transmission development.
To overcome this problem, it is well known to provide an additional gearbox which serves the sole purpose of adapting the vertical displacement of the output shaft (to which the axle is connected via a differential) relative to the input shaft. This arrangement is shown in FIG. 1 in which a tractor 1 has an engine 2 providing drive to a transmission 3 which in turn has a rear PTO shaft 4 and a rear output shaft 5. The rear out put shaft 5 drives a transfer box 6 and a rear axle 7 via a rear differential 11. The transfer box 6 has a front drive shaft 8 which drives a front axle 9. It can be seen that the transfer box 6 does not form part of the transmission 3 and serves to drop the front drive shaft 8 by a vertical distance A relative to the rear output shaft 5.
FIGS. 2A to 2D show alternative uses of the “bolt-on” transfer box 6 to alter the relative vertical displacements of the output shafts. FIG. 2A replicates part of the view of FIG. 2 and once again shows the relative vertical distance A between front drive shaft 8 and the rear output shaft 5 for ease comparison with FIGS. 2B to 2D. FIG. 2B shows the use of a second configuration of transfer box 6B which raises the height of the rear differential 11 by a distance B relative to the rear output shaft 5. FIG. 2C shows use of a second transfer box 6′ in front of the transmission 3 in order to raise the height of the front differential (not shown for clarity) by a distance C relative to the front drive shaft 8. Lastly, FIG. 2D shows an arrangement similar to FIG. 2B in which transfer box 6D raises the height of the rear differential 11 by a distance D relative to the rear output shaft 5.
It will be evident that this solution requires additional hardware to be selectively designed and installed on the vehicle, hardware which is additional to a potentially wide range of different transmissions. This increases the cost of manufacture and the maintenance requirements of the vehicle.
It is an objective of the present invention to at least mitigate one or more of the above problems.